In the prior art, automatic gain control has been used in the head positioning servo loop of a magnetic disk storage system. The servo system uses prerecorded head position information on one surface of a stack of rotating magnetic platters to generate a regular signal for control. The information is read from a servo head flying above the servo surface. As the servo head is mechanically ganged to data heads flying over the other surfaces of the stack of rotating magnetic platters via a carriage mechanism, the servo position information indicates the position of the data heads. The head position information must be amplified, filtered and decoded (or peak detected) to provide a position error signal used to drive a linear motor attached to the carriage mechanism such that the data heads are correctly positioned over their respective data cylinders. An automatic gain control is used in the head positioning servo decoding system to remove signal amplitude variations due to media variations, head variations, amplification variations and flying height variations which otherwise cause inaccurate head positioning.
The bandwidth or response time of the automatic gain control is the ability to follow a time varying input signal. One such time varying signal is that of vertical mode instability or mechanical resonance of the servo head itself. As the servo head moves up and down with respect to the disk, the output signal from the head changes in amplitude because the output amplitude of the servo head is a function of its flying height. Thus, if the automatic gain control had a bandwidth or response time in excess of the frequency of the vertical mode instability, the amplitude variations would effectively be removed. However, this solution would also prevent the system from operating on the servo data.